imaradiostar
02-12-2008, 09:52 PM
The other week I stopped by a shop that used to sell a lot of tubes back in the day. Most of the stock is pretty picked over but I did find something kindof interesting. I bought 5 5686 miniature beam power tetrodes. They're the same pinout as an EL84 but with extra pins used to double some connections for RF use.
These little suckers are about the size and fil current of a 12ax7 but they're rated at 8 watts max dissipation and are designed to produce 2.7 watts SE into a 9k load. I couldn't find any info on running them push-pull but I did have my plexi-jr on the bench and a lower voltage PT laying around so I hooked it up and biased them at a few different settings and played a bit. I don't know what plate load to use in push-pull but it can't be too far off from commonly available parts.
I think this could make a really interesting low volume push-pull tube. Using roughly 6k plate load in push pull it was darn quiet; probably not even as loud as a valve junior. I'm going to try some different output transformers and see what they'll do but I bet this would be an excellent solution for someone wanting to do that low volume thing but with a really low power pentode rather than a 6sn7 or 12au7 or similar. They require a substantial amount of input to produce a decent output, similar to a 6v6.
How did they sound? Well, I'm used to hearing that amp with EL84's, fixed bias and plate voltages around 375. Usually it's very marshally. With the 5686's and smaller PT I had 250-260 volts on the plates. I tried bias between 15 and 30 ma per tube. These things have pretty low transconductance so it took a lot of voltage at the grids to get them to bias correctly. Colder bias sounded cleaner and slightly brighter- better to my ears but your mileage may vary. It didn't sound like a 6v6 which is a welcome change to my ears! I'm hoping for a "baby kt66" sound after some tweaking.
More testing to follow if anyone is interested...let me know.
jamie
These little suckers are about the size and fil current of a 12ax7 but they're rated at 8 watts max dissipation and are designed to produce 2.7 watts SE into a 9k load. I couldn't find any info on running them push-pull but I did have my plexi-jr on the bench and a lower voltage PT laying around so I hooked it up and biased them at a few different settings and played a bit. I don't know what plate load to use in push-pull but it can't be too far off from commonly available parts.
I think this could make a really interesting low volume push-pull tube. Using roughly 6k plate load in push pull it was darn quiet; probably not even as loud as a valve junior. I'm going to try some different output transformers and see what they'll do but I bet this would be an excellent solution for someone wanting to do that low volume thing but with a really low power pentode rather than a 6sn7 or 12au7 or similar. They require a substantial amount of input to produce a decent output, similar to a 6v6.
How did they sound? Well, I'm used to hearing that amp with EL84's, fixed bias and plate voltages around 375. Usually it's very marshally. With the 5686's and smaller PT I had 250-260 volts on the plates. I tried bias between 15 and 30 ma per tube. These things have pretty low transconductance so it took a lot of voltage at the grids to get them to bias correctly. Colder bias sounded cleaner and slightly brighter- better to my ears but your mileage may vary. It didn't sound like a 6v6 which is a welcome change to my ears! I'm hoping for a "baby kt66" sound after some tweaking.
More testing to follow if anyone is interested...let me know.
jamie